Capacitive-type acceleration sensor

ABSTRACT

A capacitive-type acceleration sensor includes a sensor chip that forms a moving electrode and a fixed electrode. The electrodes face each other while maintaining a detection gap. The sensor chip is joined to a circuit chip integrally. The circuit chip is mounted on a package via a resin adhesive. The adhesive is mixed with a filler of a material having a Young&#39;s modulus higher than that of the resin.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on and incorporates herein by referenceJapanese Patent Application No. 2002-225701 filed on Aug. 2, 2002.

FIELD OF THE INVENTION

[0002] This invention relates to a capacitive-type acceleration sensorequipped with a sensor chip that has a moving electrode and a fixedelectrode.

BACKGROUND OF THE INVENTION

[0003] A capacitive-type acceleration sensor is equipped with a sensorchip having a moving electrode and a fixed electrode formed in anopposing manner on a semiconductor substrate maintaining a detectiongap. When acceleration is applied in this sensor chip, the movingelectrode and the fixed electrode vibrate relative to each other,whereby the opposing distance between the two electrodes varies. Theacceleration that is applied is detected based upon a change in thecapacitance between the two electrodes accompanying the variation in thedistance.

[0004] In the acceleration sensor of this type, it is proposed to join asensor chip and a circuit chip having a function such as processing thesensor output, and attach these two chips onto a package such as ofceramics via a resin adhesive.

[0005] When the acceleration is applied in this proposed structure,however, the adhesive works as a spring and the assembled structure onthe adhesive resonates, whereby the vibration affects the vibration ofthe moving and fixed electrodes in the sensor chip, i.e., affects thevibration of the sensing unit.

[0006]FIG. 4 is a diagram schematically illustrating vibration frequencycharacteristic in the proposed capacitive-type acceleration sensor,wherein the abscissa represents the frequency f and the ordinaterepresents the resonance magnification K with which the vibration isamplified by the sensing unit and by the adhesive. FIG. 5 is a diagramschematically illustrating a relationship between the vibrationfrequency f and the amplitude magnification K of vibration in thesensor.

[0007] In this proposed sensor, the sensing unit in the sensor chip hasa predetermined resonance frequency fa, and the resonance frequencycharacteristic of the sensing unit is represented by a curve A in FIG.4. The resonance frequency fa of the sensing unit is utilized todetermine the frequency used for the measurement, i.e., to determine therange of frequencies Rf to be measured.

[0008] The external G is the external acceleration applied to the sensorfrom the outer side. The external acceleration is applied to the sensoras vibration of a magnification of 1. When the vibration system is notresonating with the adhesive as a spring, the external acceleration isamplified based on the vibration frequency characteristic of the sensingunit. In this instance, desired output characteristic is defined asrepresented by a solid-line curve L1 in FIG. 5.

[0009] When a general resin adhesive is used, however, the resonancefrequency fb of the vibration system with the adhesive as a springapproaches the resonance frequency fa of the sensing unit as representedby a curve B in FIG. 4.

[0010] Therefore, even if external acceleration is applied as vibrationof the magnification of 1, an amplified vibration of externalacceleration due to the resonance of the vibration system with theadhesive as a spring is given to the assembly structure, i.e., given tothe sensor chip and to the circuit chip.

[0011] Thus, the sensing unit detects an acceleration that is greaterthan an external acceleration that is really applied. As a result, asrepresented by a broken-line curve L2 in FIG. 5, there is produced asensor output greater than the external acceleration that is reallyapplied, and an output error occurs relative to the desired outputcharacteristic L1.

[0012] Thus, the output error increases as the resonance frequency fb ofthe vibration system with the adhesive as a spring approaches theresonance frequency fa of the sensing unit.

SUMMARY OF THE INVENTION

[0013] In view of the above problems, therefore, it is an object of thepresent invention to improve a capacitive-type acceleration sensor inwhich a sensor chip forming a moving electrode and a fixed electrode isjoined to a circuit chip integrally together, and the circuit chip ismounted on a package via a resin adhesive. Specifically it is an objectto suppress an error in the sensor output caused by the amplification ofvibration of the sensing unit due to the resonance of the vibrationsystem with the adhesive as a spring.

[0014] The present invention is based on inventor's findings that thatthe resonance frequency of the vibration system with the adhesive as aspring increases with an increase in the rigidity of the resin adhesive,i.e., increases with an increase in the Young's modulus of the adhesive,and can be separated away from the resonance frequency of the sensingunit.

[0015] According to the present invention, a capacitive-typeacceleration sensor has a sensor chip forming a moving electrode and afixed electrode facing each other while maintaining a detection gap.This sensor chip is joined to a circuit chip integrally together, andthe circuit chip is mounted on a package via a resin adhesive. Theadhesive is mixed with a filler of a material having a Young's modulushigher than that of the resin.

[0016] With the adhesive being mixed with a filler of a material havinga Young's modulus higher than that of the resin, then, the Young'smodulus can be increased in the adhesive as a whole.

[0017] Namely, as shown in FIG. 6A, the resonance frequency fb of thevibration system with the adhesive as a spring can be increased so as tobe separated away from the resonance frequency fa of the sensing unit.It is thus made possible to suppress an error in the sensor outputcaused by the amplification of vibration of the sensing unit stemmingfrom the resonance of the vibration system with the adhesive as aspring.

[0018] Namely, as shown in FIG. 6B, the amplitude magnification ofvibration of the sensing unit represented by a broken-line curveapproaches the desired output characteristic represented by a solid-linecurve to suppress an error in the sensor output.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

[0020]FIG. 1 is a sectional view schematically illustrating acapacitive-type acceleration sensor according to an embodiment of thepresent invention;

[0021]FIG. 2 is a graph schematically illustrating a relationshipbetween the Young's modulus of the adhesive and the resonance frequencyof the embodiment;

[0022]FIGS. 3A to 3C are diagrams illustrating various patterns forarranging adhesives in the embodiment;

[0023]FIG. 4 is a graph schematically illustrating vibration frequencycharacteristic of a capacitive-type acceleration sensor;

[0024]FIG. 5 is a graph schematically illustrating a relationshipbetween the vibration frequency and the amplitude magnification ofvibration in a capacitive-type acceleration sensor; and

[0025]FIGS. 6A and 6B are graphs illustrating the effect of theembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Referring to FIG. 1, a capacitive-type acceleration sensor S1 hasa sensor chip 10 formed of a semiconductor substrate such as of siliconby way of a known semiconductor process. This sensor S1 uses an SOI(silicon-on-insulator) substrate obtained by joining a second siliconsubstrate 12 onto a first silicon substrate 11 via an oxide film 13.

[0027] In the sensor chip 10 of this embodiment, grooves are formed inthe second silicon substrate 12 to sectionalize the moving electrodes 14and the fixed electrodes 15 via the grooves. The moving and fixedelectrodes 14 and 15, respectively, may be formed in a well-known combteeth-like beam structure.

[0028] Detection gaps 16 are maintained between the moving electrodes 14and the fixed electrodes 15; i.e., the two electrodes 14 and 15 arefacing each other via the detection gap 16, and a detection capacitanceis formed between the detection gaps 14 and 15.

[0029] When the acceleration is applied, therefore, the two electrodes14 and 15 vibrate relative to each other, whereby the opposing distancebetween the two electrodes 14 and 15 or the detection gap 16 varies, andthe applied acceleration is detected based upon a change in thecapacitance between the two electrodes 14 and 15 caused by a variationin the distance. Thus, the moving and fixed electrodes 14 and 15 areconstituted as a sensing unit.

[0030] In the sensor chip 10, further, the first silicon substrate 11and the oxide film 13 are removed from the portions corresponding to themoving and fixed electrodes 14 and 15, thereby to form a cavity 17. Themoving and fixed electrodes 14, 15 are liberated in an upper part of thecavity 17 so as to vibrate relative to each other.

[0031] In the sensor chip 10 of this embodiment, for example, groovesare formed from, the surface of the second silicon substrate 12 bytrench etching to define the moving and fixed electrodes 14 and 15,while the cavity 17 is formed from the side of the first siliconsubstrate 11 by anisotropic etching of silicon.

[0032] The sensor chip 10 is joined to the circuit chip 30 via anadhesive film 20 having a high rigidity. The circuit chip 30 has afunction to process the output signals from the sensor chip 10 and tosend them to an external unit, and is made of a semiconductor substratesuch as silicon. The adhesive film 20 may be a dicing tape of a resinused for dice-cutting.

[0033] The circuit chip 30 to which the sensor chip 10 is integrallyjoined is mounted on a package 50 via a resin adhesive 40. Here, theadhesive 40 is applied and cured to accomplish the adhesion. The package50 is made of ceramics and is mounted on a body to be measured, such asan automobile.

[0034] When the acceleration is applied to the capacitive-typeacceleration sensor S1, a change occurs in the capacitance in thesensing unit, i.e., between the moving and fixed electrodes 14 and 15,and the applied acceleration is detected based upon the change in thecapacitance. Here, vibration occurs with the adhesive 40 as a spring andwith the assembled structure including the circuit chip 30 and thesensor chip 10 as a mass unit.

[0035] In order to prevent the vibration of the moving and fixedelectrodes 14 and 15 from being amplified by the resonance of thevibration system with the adhesive as a spring according to thisembodiment, the adhesive 40 is mixed with filler of a material having aYoung's modulus larger than that of the resin.

[0036] The filler are used for the following reasons. In the vibrationsystem with the adhesive 40 as a spring, it has been known that theresonance frequency fb of the assembled structure (10, 30) which is themass unit (i) varies in proportion to the one-half power of the area ofadhesion of the adhesive 40, (ii) varies in proportion to the two-thirdspower of the thickness of the adhesive agent 40, and (iii) varies inproportion to the one-half power of the Young's modulus E of theadhesive 40.

[0037] Among them, the above (i) and (ii) face relatively muchlimitation from the standpoint of the shape and size of the sensor andmaintaining the adhesion strength. It is therefore contrived to increasethe Young's modulus E of the adhesive agent 40. FIG. 2 is a diagramschematically illustrating a relationship between the Young's modulus Eof the adhesive 40 and the resonance frequency fb of the assembledstructure (10, 30). As understood from FIG. 2, the resonance frequencyfb of the assembled structure increases with an increase in the Young'smodulus E of the adhesive agent.

[0038] As shown in FIG. 4, the resonance frequency of the assembledstructure is normally close to the resonance frequency fa of the movingand fixed electrodes 14, 15. In this embodiment, however, the resonancefrequency fb of the assembled structure is increased by increasing theYoung's modulus E of the adhesive in order to separate the two resonancefrequencies fa and fb away from each other.

[0039] If the adhesive 40 is mixed with filler of a material having aYoung's modulus higher than that of the resin of the adhesive, then, theYoung's modulus of the adhesive 40 as a whole can be increased.

[0040] Therefore, since the resonance frequency fb of the vibrationsystem with the adhesive 40 as a spring can be increased to be more awayfrom the resonance frequency fa of the fixed and moving electrodes 14and 15, the amplification of vibration of the moving and fixedelectrodes 14 and 15 caused by the resonance of the vibration systemwith the adhesive 40 as a spring can be suppressed.

[0041] Thus, an error in the sensor output is suppressed. Namely, theembodiment makes it possible to obtain output characteristic closer tothe desired output characteristic represented by the solid-line curve L1in FIG. 5. By separating the two resonance frequencies fa and fb awayfrom each other, the amplitude magnification K of vibration in thesensing unit of the embodiment can be brought close to the desiredoutput characteristic represented by the solid-line curve, and an errorin the sensor output can be suppressed as illustrated in FIG. 6B.

[0042] In the adhesive 40 mixed with filler of a material having aYoung's modulus higher than that of the resin of the adhesive, an epoxyresin can be used as the resin, and a metal or a glass can be used asthe filler or, more specifically, silver filler can be used. The weightratio of the filler can be selected to be, for example, about severaltens of-percent.

[0043] It is, of course, allowable to increase the Young's modulus ofthe adhesive 40 by varying the molecular weight by changing the degreeof polymerization of the resin in the adhesive 40.

[0044] As the patterns for arranging the adhesive 40, various patternsare proposed. The adhesives 40 may be arranged at four corners on thecircuit chip 30 as shown in FIG. 3A. The adhesives 40 may be arranged ina crossing manner as shown in FIG. 3B. The adhesive 40 may be arrangedon the circuit chip 30 at one central point as shown in FIG. 3C.

[0045] Among these patterns, the adhesion at four corners or theadhesion in a crossing manner encounters such a difficulty that thecorner portions of the circuit chip 30 or of the sensor chip 10 undergoa displacement due to the thermal stress caused by a change in thetemperature, and the sensor chip 10 is deformed to a large extent. Withthe central one-point adhesion, on the other hand, the corners of thecircuit chip 30 are liberated from the package 50 and, hence, thedeformation of the sensor chip 10 by the thermal stress is small.

[0046] When the sensor chip 10 deforms due to the thermal stress, theopposing area between the moving electrode 14 and the fixed electrode 15fluctuates to adversely affect the temperature characteristic of thesensor. In this regard, it is desired that the adhesive 40 is arrangedat the central portion of the circuit chip 30 like the central one-pointadhesion.

[0047] In arranging the adhesive 40, it may often be difficult to applythe adhesive 40 in compliance with the pattern. In such a case, theadhesion surfaces of the circuit chip 30 or of the package 50 may beformed rugged to realize any adhesion pattern.

[0048] The present invention should not be limited to the disclosedembodiment, but may be implemented in many other ways without departingfrom the spirit of the invention.

What is claimed is:
 1. A capacitive-type acceleration sensor comprising:a sensor chip forming a moving electrode and a fixed electrode facingeach other while maintaining a detection gap; a circuit chip jointed tothe sensor chip integrally; a package; and an adhesive provided toattach the circuit chip to the package, characterized in that theadhesive is a mixture of resin and filler mixed with the resin andhaving a Young's modulus higher than that of the resin.
 2. Acapacitive-type acceleration sensor according to claim 1, wherein thefiller is a metal or a glass.
 3. A capacitive-type acceleration sensoraccording to claim 2, wherein the filler is silver.
 4. A capacitive-typeacceleration sensor according to claim 1, wherein the adhesive isarranged at only a central portion of the circuit chip.
 5. Acapacitive-type acceleration sensor comprising: a sensor chip forming amoving electrode and a fixed electrode facing each other whilemaintaining a detection gap to capacitively sense acceleration appliedthereto, the sensor chip having a first resonance frequency; a package;and a resin adhesive provided to attach the sensor chip to the package,the resin adhesive operating as a spring to form a resonance systemhaving a second resonance frequency higher than the first frequency,wherein the resin adhesive includes a filler to increase the secondresonance frequency than when no filler is mixed.